A unit cell of a fuel cell system includes a membrane electrode assembly (MEA) that is sandwiched by and between separators. When the separators are in the form of metal separators, a structural material (such as a resin frame) may be interposed between the MEA and each of the separators. A plurality of such unit cells are stacked together to form a fuel cell stack. A cell voltage monitor (which will be hereinafter called “cell monitor”) for measuring voltage is mounted on each cell (or unit cell) or a suitable number of cells of the fuel cell stack. As disclosed in JP-A-2004-63176, or as shown in FIG. 7, a cell monitor 1 is mounted on a fuel cell such that only one of a pair of separators 2 of a unit cell is sandwiched by the cell monitor 1.
However, the cell monitor mounting structure as described above has the following problem. Since the cell monitor is mounted on and held by a single sheet of separator, the separator may be bent or broken if excessively large external force is applied to the cell monitor, and it is thus necessary to provide a cell monitor mounting portion of the cell with high rigidity and high strength. In the case where the separator comprises a thin metal separator, in particular, the separator is likely to be deformed or bent to be broken, and, therefore, it is highly desired to improve the rigidity and strength of the cell monitor mounting portion of the fuel cell.